Depolarizers are widely used in products that incorporate optical fiber, i.e. fiber optic gyroscopes, and play a major role in sensor technology. Because depolarizers can increase the performance and quality of a product containing optical fiber, depolarizers are important to fiber optic gyro systems. Specifically, because gyro error signals arise as a result of different polarization conditions of the individual light wave trains within the optical fiber, depolarizers can is be used to suppress gyro error.
A common technique for fabricating a depolarizer involves aligning the optical axes of two birefringent fibers at 45.degree.. To determine a successful 45.degree. alignment, an analyzer measures the polarization extinction ratio at the output of the fiber. When the measured intensity is independent of the analyzer's angular position with respect to the second fiber, a 45.degree. alignment is achieved. The extinction ratio designates the ratio of the intensities contained in any two orthogonal axes of the birefringent fiber (dB=10 logI.sub.min /I.sub.max !). The decibel (dB) is a customary unit for designating the ratio. An optimum polarizer requires a 90 dB extinction ratio, while an optimum depolarizer requires a 0 dB extinction ratio.
After aligning a broad band polarized input source, the optimum depolarizer would have an output with an equal amount of uncorrelated light in each of the two orthogonal axes, and thus, the extinction ratio equal to 0 dB because I.sub.min =I.sub.max and log1!=0. To achieve an equal amount of light in each axis, the axes of two fiber segments must be aligned at 45.degree. relative to each other. An azimuthal splice orientation alignment of 45.degree. would transfer incoming polarized light into equal orthogonal electric fields (E.sub.max and E.sub.min are equal electric field components in each axis). An alignment not equaling 45.degree. would result an increased intensity along one axis and a decreased intensity along the other axis.
In current methods, when fabricating a depolarizer, the output photodetector is configured with an analyzer to measure the maximum and minimum output intensities (e.g. I.sub.min and I.sub.max are the intensities along the fast and slow axis) of the coupled fiber. That is, a PM fiber is cleaved so that the resulting fiber segments are long enough for source depolarization. Arbitrarily polarized light is coupled into the first fiber. The second fiber is aligned with the first fiber. I.sub.min and I.sub.max are determined by locating the maximum and minimum intensity axes at the end of the second fiber. The first fiber is rotated with respect to the second fiber, then the analyzer is rotated to determine the extinction ratio from I.sub.max and I.sub.min. If the extinction ratio is 0 dB, the fiber segments are fused to form a depolarizer. If the extinction ratio is not 0 dB, the first fiber must again be rotated until the intensities are equal in each axes, then the analyzer is again rotated to determine if a 0 dB extinction ratio has been achieved. Iteratively rotating the first fiber segment and analyzer requires extra skill and time and is susceptible to measurement errors.
If, when the light enters the first fiber, the light is not entirely on one birefringent axes, the light will be at least partially decorrelated prior to the splice alignment. Therefore, because of preexisting decorrelation, determining the quality of the splice angle alignment is difficult. More particularly, if the light enters the first fiber at 45.degree. from the fast and slow axes, the light will decorrelate even if no splice misalignment existed. Optimally, instead of coupling the light into the first fiber at any random angle, the light enters the fiber on one birefringent axis. Light entering the fiber on one axis will remain correlated with respect to itself throughout the entire fiber. Thus, light entering on one axis places the burden of distribution between axes solely on the 45.degree. splice. Complete decorrelation (or depolarization) requires an exact 45.degree. splice.
Many methods for depolarizing light exist, but each of the currently known methods suffer from important disadvantages making them difficult to use, unreliable and/or expensive. The Laskoskie, et al. patent, U.S. Pat. No. 5,351,124, discloses an appropriate system for aligning the birefringent axes through the use of a temporary third fiber and an interferometer. The '124 patent recognizes the problems with the prior art systems; however, the solution proposed in the '124 patent requires the use of expensive, complicated components, such as an interferometer, which require special skills for its operation.
To overcome the iterative rotation problems of the prior art, the Michal patent (U.S. Pat. No. 5,486,916) discloses an apparatus for aligning the birefringent axes through the use of a heated fiber coil. A low birefringent fiber sensing coil is connected to one of the output circuits and a reciprocal interferometer is also used to measure the alignment of the two fibers. By incorporating a heating coil and reciprocal interferometer, the '916 patent requires the use of expensive, complicated devices for the fabrication of a depolarizer.
Other systems that exist include a method that has been used to produce effectively unpolarized light is to split a polarized beam into a plurality of subbeams and then recombine them. The recombination produces a varying pattern of polarization states across the face of a detector to form a spatial average. This method is not useful with single mode fibers because it involves a spatial average across a comparatively large area.
Yet another way of making a depolarizer is through the use of AC detection. The AC signal is generated by birefringence modulation. The modulation disappears when the axes of both fibers are rotated to 45.degree. with respect to each other. However, the AC detection method requires high voltages to operate a Pockels cell which is not only dangerous and costly, but also a complex optical circuit arrangement.
The prior art indicates that while conventional depolarizer fabrication methods exist, they each suffer from the principal disadvantage of requiring complicated devices and iterative measurements. Thus, a system and method is needed for providing a depolarizer which overcomes the shortcomings of the prior art. Therefore, a long-felt need exists to ameliorate the disadvantages occasioned by the known fabrication techniques of depolarizers in a more efficient, accurate and cost-effective manner to overcome the prior art.